A GP100 is a 357Magnum. It can therefore eat all 357Magnum, 38Special and 38Special "+P" and "+P+" ammo.
The max pressure spec for 38Spl is about 18,000psi. "+P" means extra pressure, generally about 10% over that tops. "+P+" is "God only knows" but at least somewhat less than 357Magnum pressures. (Some of it damned close to 357 levels.)
Max pressure TODAY for 357Mag is 35,000psi per SAAMI. It used to be 43,500. A few ammo houses load their top stuff at the old standard - Buffalo Bore, DoubleTap Ammo, Grizzly Ammo. The GP100 is strong enough to eat that stuff too.
It CANNOT eat 357Sig (won't fit at all), 357Maximum (too long), 38S&W (too fat).